1898 FLORA OF LOWER SONORAN AND ARID ZONES 123 
above 3000", in the latter of which a very prominent Mexican 
highland and boreal element occurs. 
The vegetation with which we have here to deal is above all 
one of xerophytic stamp. It falls into the two following cate- 
gories: (1) groups distributed more or less continuously along 
the continental axis, always interrupted by the moist tropical 
and subtropical belt from south Mexico to the Andes of Colom- 
bia and Venezuela (the subandine province of Engler‘), but 
with a very marked development in the arid plains and plateaus 
of the two extra-tropical zones in question; (2) groups not at 
all high mountain plants, but existing both in the Chilian or 
Argentine arid zones and in the Lower Sonoran zone of North 
America, but entirely disconnected through the intervening dis- 
tance. It is evident that group (2) will furnish the more inter- 
esting and difficult questions of distribution as related to gen- 
etic affinities. It is further evident that satisfactory discussion 
can follow only after a critical determination of what those affini- 
ties are. In the following pages the writer presents a great deal 
more than he has himself determined critically, recognizing also 
that some excellent illustrative groups are left unmentioned. 
The instances cited from Amarantacee, Malvacee, Loasa- 
cee, and Leguminose are not the results of my own study, 
although I have been able to compare specimens in all of them. 
The Loasaceze will be very fully treated in the forthcoming 
monograph by Drs. Urban and Gilg. Gomphrena, Malvastrum, 
Spheralcea, and Prosopis deserve more special study in compar- 
ing the species of the different regions concerned. One of the 
very best illustrations was found in the Zygophyllacee, in which, 
of course, I have simply used the results of Professor Engler’s 
Study of the family. In the other cases I have made a rather 
More detailed study of the specimens in the Berlin Herbarium. 
AMARANTACE#-GOMPHRENES. 
This group of the Amarantacee is peculiarly the New World 
development of the family, finding there a subtropical and warm 
* Entwicklungs-geschichte der Pflanzenwelt 2 : 206. 
